604 PHYSIOLOGY. 



the endoneurium. The larger blood- and lymph- vesssls lie in the 

 epineurium; the few capillaries of the nerve-fibers lie supported in 

 the endoneurium. 



BEGENERATION or UNITED NERVES. If a nerve is cut, its peri- 

 pheral end undergoes degeneration. The fiber breaks up into small 

 pieces of myelin, each holding a piece of neuraxon which is finally 

 absorbed. Eepair of the nerve begins wholly during the degenera- 

 tion. The nuclei of the neurilemma increase in number to form 

 around them a layer of protoplasm or cytoplasm. At length the 

 cytoplasm becomes a continuous piece of protoplasm, and the fiber 

 thus produced is known as a "band-fiber." Then there is an arrest of 

 regeneration unless the peripheral fiber is anatomically united to its 

 central connection. If the central and peripheral ends are brought 

 together, then the "band-fiber" becomes changed into a normal nerve- 

 fiber, with a sheet of myelin and a cylinder axis. The axis cylinder 

 in the peripheral end of the nerve is supposed to grow out from the 

 central end of the nerve. 



Termination of the Nerve. After a certain course in the trunk 

 of the nerve the nerve-fiber divides at the periphery into a terminal 

 plaque, the motor plaque of muscles; or into a sense-cell, as in the 

 retinal cells or organ of Corti ; or into a sense-corpuscle, as a tactile 

 corpuscle; or into numerous fibrils which anastomose to form a 

 terminal p'exus, as in the cornea. 



NONMEDULLATED FIBERS, that is, those that are naked, pale or 

 gray, and reduced to an axis-cylinder and sheath, branch and form 

 networks their peripheral terminations. This mode of termination 

 occurs in the nerve-fibers of common sensation, as in many of the 

 nerve-fibers of the skin, cornea, and mucous membrane. In all of 

 these cases the peripheral termination fibrils are intra-epithelial : 

 that is, they are situated in the epithelial portions of cornea, mucous 

 membrane, etc. 



Neuroglia. In the gray, as well as in the white, substance of 

 the nerve-centers there exists between the cells and nerve-fibers an 

 intervening substance which has been termed neuroglia. It must not 

 be confounded with the true connective tissue along the course of 

 the blood-vessels in the nerve-centers. Its chemical nature is wholly 

 different from the latter, which is always derived from the mesoblast. 

 Eanvier has shown that neuroglia is derived from the primitive neuro- 

 blast or epiblast. 



Neuroglia sometimes presents itself in the shape of very fine filar 

 ments assembled in a very close network, as in the gray substance. 





